Dear future visitors, the answers to this question equally apply to an ArgumentNullException. If your question has been closed as a duplicate of this one, and you are experiencing an ANE, please follow the directions in the answers to debug and fix your problem.
– WillOct 13 '17 at 17:56

@will ANE should only happen if a null is passed as a parameter. Can you give an example if an ANE question closed as a duplicate of this one?
– John SaundersOct 14 '17 at 21:44

It came up on Meta, but I'd have to go digging for the link. But as to that comment, an ANE is simply a NRE but someone added a preemptive check, and you at least know exactly what is null (the argument name is provided), so it's a little easier to diagnose than a straight up NRE.
– WillOct 16 '17 at 12:53

31 Answers
31

What is the cause?

Bottom Line

You are trying to use something that is null (or Nothing in VB.NET). This means you either set it to null, or you never set it to anything at all.

Like anything else, null gets passed around. If it is nullin method "A", it could be that method "B" passed a nullto method "A".

null can have different meanings:

Object variables which are uninitialized and hence point to nothing. In this case, if you access properties or methods of such objects, it causes a NullReferenceException.

The developer is using null intentionally to indicate there is no meaningful value available. Note that C# has the concept of nullable datatypes for variables (like database tables can have nullable fields) - you can assign null to them to indicate there is no value stored in it, for example int? a = null; where the question mark indicates it is allowed to store null in variable a. You can check that either with if (a.HasValue) {...} or with if (a==null) {...}. Nullable variables, like a this example, allow to access the value via a.Value explicitly, or just as normal via a. Note that accessing it via a.Value throws an InvalidOperationException instead of a NullReferenceException if a is null - you should do the check beforehand, i.e. if you have another on-nullable variable int b; then you should do assignments like if (a.HasValue) { b = a.Value; } or shorter if (a != null) { b = a; }.

The rest of this article goes into more detail and shows mistakes that many programmers often make which can lead to a NullReferenceException.

More Specifically

The runtime throwing a NullReferenceExceptionalways means the same thing: you are trying to use a reference, and the reference is not initialized (or it was once initialized, but is no longer initialized).

This means the reference is null, and you cannot access members (such as methods) through a null reference. The simplest case:

string foo = null;
foo.ToUpper();

This will throw a NullReferenceException at the second line because you can't call the instance method ToUpper() on a string reference pointing to null.

Debugging

How do you find the source of a NullReferenceException? Apart from looking at the exception itself, which will be thrown exactly at the location where it occurs, the general rules of debugging in Visual Studio apply: place strategic breakpoints and inspect your variables, either by hovering the mouse over their names, opening a (Quick)Watch window or using the various debugging panels like Locals and Autos.

If you want to find out where the reference is or isn't set, right-click its name and select "Find All References". You can then place a breakpoint at every found location and run your program with the debugger attached. Every time the debugger breaks on such a breakpoint, you need to determine whether you expect the reference to be non-null, inspect the variable and and verify that it points to an instance when you expect it to.

By following the program flow this way, you can find the location where the instance should not be null, and why it isn't properly set.

Examples

Some common scenarios where the exception can be thrown:

Generic

ref1.ref2.ref3.member

If ref1 or ref2 or ref3 is null, then you'll get a NullReferenceException. If you want to solve the problem, then find out which one is null by rewriting the expression to its simpler equivalent:

var r1 = ref1;
var r2 = r1.ref2;
var r3 = r2.ref3;
r3.member

Specifically, in HttpContext.Current.User.Identity.Name, the HttpContext.Current could be null, or the User property could be null, or the Identity property could be null.

Indirect

public class Person {
public int Age { get; set; }
}
public class Book {
public Person Author { get; set; }
}
public class Example {
public void Foo() {
Book b1 = new Book();
int authorAge = b1.Author.Age; // You never initialized the Author property.
// there is no Person to get an Age from.
}
}

If you want to avoid the child (Person) null reference, you could initialize it in the parent (Book) object's constructor.

Nested Object Initializers

The same applies to nested object initializers:

Book b1 = new Book { Author = { Age = 45 } };

This translates to

Book b1 = new Book();
b1.Author.Age = 45;

While the new keyword is used, it only creates a new instance of Book, but not a new instance of Person, so the Author the property is still null.

The new Person only creates an instance of Person, but the Books collection is still null. The collection initializer syntax does not create a collection
for p1.Books, it only translates to the p1.Books.Add(...) statements.

Array

Array Elements

Person[] people = new Person[5];
people[0].Age = 20 // people[0] is null. The array was allocated but not
// initialized. There is no Person to set the Age for.

Jagged Arrays

long[][] array = new long[1][];
array[0][0] = 3; // is null because only the first dimension is yet initialized.
// Use array[0] = new long[2]; first.

Collection/List/Dictionary

Dictionary<string, int> agesForNames = null;
int age = agesForNames["Bob"]; // agesForNames is null.
// There is no Dictionary to perform the lookup.

Range Variable (Indirect/Deferred)

public class Person {
public string Name { get; set; }
}
var people = new List<Person>();
people.Add(null);
var names = from p in people select p.Name;
string firstName = names.First(); // Exception is thrown here, but actually occurs
// on the line above. "p" is null because the
// first element we added to the list is null.

ASP.NET Session Values

// if the "FirstName" session value has not yet been set,
// then this line will throw a NullReferenceException
string firstName = Session["FirstName"].ToString();

ASP.NET MVC empty view models

If the exception occurs when referencing a property of @Model in an ASP.NET MVC view, you need to understand that the Model gets set in your action method, when you return a view. When you return an empty model (or model property) from your controller, the exception occurs when the views access it:

WPF Control Creation Order and Events

WPF controls are created during the call to InitializeComponent in the order they appear in the visual tree. A NullReferenceException will be raised in the case of early-created controls with event handlers, etc. , that fire during InitializeComponent which reference late-created controls.

Use Debug.Assert if a value should never be null, to catch the problem earlier than the exception occurs.

When you know during development that a method maybe can, but never should return null, you can use Debug.Assert() to break as soon as possible when it does occur:

string GetTitle(int knownBookID) {
// You know this should never return null.
var book = library.GetBook(knownBookID);
// Exception will occur on the next line instead of at the end of this method.
Debug.Assert(book != null, "Library didn't return a book for known book ID.");
// Some other code
return book.Title; // Will never throw NullReferenceException in Debug mode.
}

Use the null condition operator: ?. or ?[x] for arrays (available in C# 6 and VB.NET 14):

This is also sometimes called the safe navigation or Elvis (after its shape) operator. If the expression on the left side of the operator is null, then the right side will not be evaluated, and null is returned instead. That means cases like this:

var title = person.Title.ToUpper();

If the person does not have a title, this will throw an exception because it is trying to call ToUpper on a property with a null value.

In C# 5 and below, this can be guarded with:

var title = person.Title == null ? null : person.Title.ToUpper();

Now the title variable will be null instead of throwing an exception. C# 6 introduces a shorter syntax for this:

var title = person.Title?.ToUpper();

This will result in the title variable being null, and the call to ToUpper is not made if person.Title is null.

Of course, you still have to check title for null or use the null condition operator together with the null coalescing operator (??) to supply a default value:

This will do the following: If myIntArray is null, the expression returns null and you can safely check it. If it contains an array, it will do the same as:
elem = myIntArray[i]; and returns the ith element.

Special techniques for debugging and fixing null derefs in iterators

C# supports "iterator blocks" (called "generators" in some other popular languages). Null dereference exceptions can be particularly tricky to debug in iterator blocks because of deferred execution:

If whatever results in null then MakeFrob will throw. Now, you might think that the right thing to do is this:

// DON'T DO THIS
public IEnumerable<Frob> GetFrobs(FrobFactory f, int count)
{
if (f == null)
throw new ArgumentNullException("f", "factory must not be null");
for (int i = 0; i < count; ++i)
yield return f.MakeFrob();
}

Why is this wrong? Because the iterator block does not actually run until the foreach! The call to GetFrobs simply returns an object which when iterated will run the iterator block.

By writing a null check like this you prevent the null dereference, but you move the null argument exception to the point of the iteration, not to the point of the call, and that is very confusing to debug.

That is, make a private helper method that has the iterator block logic, and a public surface method that does the null check and returns the iterator. Now when GetFrobs is called, the null check happens immediately, and then GetFrobsForReal executes when the sequence is iterated.

If you examine the reference source for LINQ to Objects you will see that this technique is used throughout. It is slightly more clunky to write, but it makes debugging nullity errors much easier. Optimize your code for the convenience of the caller, not the convenience of the author.

A note on null dereferences in unsafe code

C# has an "unsafe" mode which is, as the name implies, extremely dangerous because the normal safety mechanisms which provide memory safety and type safety are not enforced. You should not be writing unsafe code unless you have a thorough and deep understanding of how memory works.

In unsafe mode, you should be aware of two important facts:

dereferencing a null pointer produces the same exception as dereferencing a null reference

dereferencing an invalid non-null pointer can produce that exception
in some circumstances

To understand why that is, it helps to understand how .NET produces null dereference exceptions in the first place. (These details apply to .NET running on Windows; other operating systems use similar mechanisms.)

Memory is virtualized in Windows; each process gets a virtual memory space of many "pages" of memory that are tracked by the operating system. Each page of memory has flags set on it which determine how it may be used: read from, written to, executed, and so on. The lowest page is marked as "produce an error if ever used in any way".

Both a null pointer and a null reference in C# are internally represented as the number zero, and so any attempt to dereference it into its corresponding memory storage causes the operating system to produce an error. The .NET runtime then detects this error and turns it into the null dereference exception.

That's why dereferencing both a null pointer and a null reference produces the same exception.

What about the second point? Dereferencing any invalid pointer that falls in the lowest page of virtual memory causes the same operating system error, and thereby the same exception.

Why does this make sense? Well, suppose we have a struct containing two ints, and an unmanaged pointer equal to null. If we attempt to dereference the second int in the struct, the CLR will not attempt to access the storage at location zero; it will access the storage at location four. But logically this is a null dereference because we are getting to that address via the null.

If you are working with unsafe code and you get a null dereference exception, just be aware that the offending pointer need not be null. It can be any location in the lowest page, and this exception will be produced.

Maybe this is a dumb comment but wouldnt the first and best way to avoid this problem be to initialize the object? For me if this error occurs it is usually because I forgot to initialize something like the array element. I think it is far less common to define the object as null and then reference it. Maybe give the way to solve each problem adjacent to the description. Still a good post.
– JPKMay 20 '14 at 6:39

29

What if there is no object, but rather the return value from a method or property?
– John SaundersMay 20 '14 at 6:41

6

The book/author example is a little weird.... How does that even compile? How does intellisense even work? What is this I'm not good with computar...
– WillSep 8 '14 at 18:26

5

@Will: does my last edit help? If not, then please be more explicit about what you see as a problem.
– John SaundersSep 8 '14 at 18:41

6

@JohnSaunders Oh, no, sorry, I meant the object initializer version of that. new Book { Author = { Age = 45 } }; How does the inner initialization even... I can't think of a situation where inner init would ever work, yet it compiles and intellisense works... Unless for structs?
– WillSep 8 '14 at 18:44

NullReference Exception — Visual Basic

The NullReference Exception for Visual Basic is no different from the one in C#. After all, they are both reporting the same exception defined in the .NET Framework which they both use. Causes unique to Visual Basic are rare (perhaps only one).

This answer will use Visual Basic terms, syntax, and context. The examples used come from a large number of past Stack Overflow questions. This is to maximize relevance by using the kinds of situations often seen in posts. A bit more explanation is also provided for those who might need it. An example similar to yours is very likely listed here.

Note:

This is concept-based: there is no code for you to paste into your project. It is intended to help you understand what causes a NullReferenceException (NRE), how to find it, how to fix it, and how to avoid it. An NRE can be caused many ways so this is unlikely to be your sole encounter.

The examples (from Stack Overflow posts) do not always show the best way to do something in the first place.

Typically, the simplest remedy is used.

Basic Meaning

The message "Object not set to an instance of Object" means you are trying to use an object which has not been initialized. This boils down to one of these:

Your code declared an object variable, but it did not initialize it (create an instance or 'instantiate' it)

Something which your code assumed would initialize an object, did not

Possibly, other code prematurely invalidated an object still in use

Finding The Cause

Since the problem is an object reference which is Nothing, the answer is to examine them to find out which one. Then determine why it is not initialized. Hold the mouse over the various variables and Visual Studio (VS) will show their values - the culprit will be Nothing.

You should also remove any Try/Catch blocks from the relevant code, especially ones where there is nothing in the Catch block. This will cause your code to crash when it tries to use an object which is Nothing. This is what you want because it will identify the exact location of the problem, and allow you to identify the object causing it.

A MsgBox in the Catch which displays Error while... will be of little help. This method also leads to very bad Stack Overflow questions, because you can't describe the actual exception, the object involved or even the line of code where it happens.

You can also use the Locals Window (Debug -> Windows -> Locals) to examine your objects.

Once you know what and where the problem is, it is usually fairly easy to fix and faster than posting a new question.

Note: Do not use Dim again in a procedure, including the constructor (Sub New):

Private reg As CashRegister
'...
Public Sub New()
'...
Dim reg As New CashRegister
End Sub

This will create a local variable, reg, which exists only in that context (sub). The reg variable with module level Scope which you will use everywhere else remains Nothing.

Missing the New operator is the #1 cause of NullReference Exceptions seen in the Stack Overflow questions reviewed.

Visual Basic tries to make the process clear repeatedly using New: Using the New Operator creates a new object and calls Sub New -- the constructor -- where your object can perform any other initialization.

To be clear, Dim (or Private) only declares a variable and its Type. The Scope of the variable - whether it exists for the entire module/class or is local to a procedure - is determined by where it is declared. Private | Friend | Public defines the access level, not Scope.

Using a List(Of T) will make it quite difficult to have an element without a valid object:

Dim FooList As New List(Of Foo) ' List created, but it is empty
Dim f As Foo ' Temporary variable for the loop
For i As Integer = 0 To 5
f = New Foo() ' Foo instance created
f.Bar = i * 10
FooList.Add(f) ' Foo object added to list
Next

Lists and Collections

You get the same exception for the same reason - myList was only declared, but no instance created. The remedy is the same:

myList = New List(Of String)
' Or create an instance when declared:
Private myList As New List(Of String)

A common oversight is a class which uses a collection Type:

Public Class Foo
Private barList As List(Of Bar)
Friend Function BarCount As Integer
Return barList.Count
End Function
Friend Sub AddItem(newBar As Bar)
If barList.Contains(newBar) = False Then
barList.Add(newBar)
End If
End Function

Either procedure will result in an NRE, because barList is only declared, not instantiated. Creating an instance of Foo will not also create an instance of the internal barList. It may have been the intent to do this in the constructor:

Public Sub New ' Constructor
' Stuff to do when a new Foo is created...
barList = New List(Of Bar)
End Sub

As before, this is incorrect:

Public Sub New()
' Creates another barList local to this procedure
Dim barList As New List(Of Bar)
End Sub

Data Provider Objects

Working with databases presents many opportunities for a NullReference because there can be many objects (Command, Connection, Transaction, Dataset, DataTable, DataRows....) in use at once. Note: It does not matter which data provider you are using -- MySQL, SQL Server, OleDB, etc. -- the concepts are the same.

As before, the ds Dataset object was declared, but an instance was never created. The DataAdapter will fill an existing DataSet, not create one. In this case, since ds is a local variable, the IDE warns you that this might happen:

When declared as a module/class level variable, as appears to be the case with con, the compiler can't know if the object was created by an upstream procedure. Do not ignore warnings.

A typo is a problem here: Employees vs Employee. There was no DataTable named "Employee" created, so a NullReferenceException results trying to access it. Another potential problem is assuming there will be Items which may not be so when the SQL includes a WHERE clause.

Remedy

Since this uses one table, using Tables(0) will avoid spelling errors. Examining Rows.Count can also help:

If ds.Tables(0).Rows.Count > 0 Then
txtID.Text = ds.Tables(0).Rows(0).Item(1)
txtID.Name = ds.Tables(0).Rows(0).Item(2)
End If

Fill is a function returning the number of Rows affected which can also be tested:

If da.Fill(ds, "Employees") > 0 Then...

Example 3

Dim da As New OleDb.OleDbDataAdapter("SELECT TICKET.TICKET_NO,
TICKET.CUSTOMER_ID, ... FROM TICKET_RESERVATION AS TICKET INNER JOIN
FLIGHT_DETAILS AS FLIGHT ... WHERE [TICKET.TICKET_NO]= ...", con)
Dim ds As New DataSet
da.Fill(ds)
If ds.Tables("TICKET_RESERVATION").Rows.Count > 0 Then

The DataAdapter will provide TableNames as shown in the previous example, but it does not parse names from the SQL or database table. As a result, ds.Tables("TICKET_RESERVATION") references a non-existent table.

AndAlso is important. Subsequent tests will not be performed once the first False condition is encountered. This allows the code to safely 'drill' into the object(s) one 'level' at a time, evaluating myFoo.Bar only after (and if) myFoo is determined to be valid. Object chains or paths can get quite long when coding complex objects:

myBase.myNodes(3).Layer.SubLayer.Foo.Files.Add("somefilename")

It is not possible to reference anything 'downstream' of a null object. This also applies to controls:

Among other things, this code does not anticipate that the user may not have selected something in one or more UI controls. ListBox1.SelectedItem may well be Nothing, so ListBox1.SelectedItem.ToString will result in an NRE.

Remedy

Validate data before using it (also use Option Strict and SQL parameters):

This is a fairly common way to get an NRE. In C#, depending on how it is coded, the IDE will report that Controls does not exist in the current context, or "cannot reference non-static member". So, to some extent, this is a VB-only situation. It is also complex because it can result in a failure cascade.

The arrays and collections cannot be initialized this way. This initialization code will run before the constructor creates the Form or the Controls. As a result:

Lists and Collection will simply be empty

The Array will contain five elements of Nothing

The somevar assignment will result in an immediate NRE because Nothing doesn't have a .Text property

Referencing array elements later will result in an NRE. If you do this in Form_Load, due to an odd bug, the IDE may not report the exception when it happens. The exception will pop up later when your code tries to use the array. This "silent exception" is detailed in this post. For our purposes, the key is that when something catastrophic happens while creating a form (Sub New or Form Load event), exceptions may go unreported, the code exits the procedure and just displays the form.

Since no other code in your Sub New or Form Load event will run after the NRE, a great many other things can be left uninitialized.

It is curious that VB does not provide a warning, but the remedy is to declare the containers at the form level, but initialize them in form load event handler when the controls do exist. This can be done in Sub New as long as your code is after the InitializeComponent call:

The array code may not be out of the woods yet. Any controls which are in a container control (like a GroupBox or Panel) will not be found in Me.Controls; they will be in the Controls collection of that Panel or GroupBox. Nor will a control be returned when the control name is misspelled ("TeStBox2"). In such cases, Nothing will again be stored in those array elements and an NRE will result when you attempt to reference it.

These should be easy to find now that you know what you are looking for:

"Button2" resides on a Panel

Remedy

Rather than indirect references by name using the form's Controls collection, use the control reference:

Function Returning Nothing

Private bars As New List(Of Bars) ' Declared and created
Public Function BarList() As List(Of Bars)
bars.Clear
If someCondition Then
For n As Integer = 0 to someValue
bars.Add(GetBar(n))
Next n
Else
Exit Function
End If
Return bars
End Function

This is a case where the IDE will warn you that 'not all paths return a value and a NullReferenceException may result'. You can suppress the warning, by replacing Exit Function with Return Nothing, but that does not solve the problem. Anything which tries to use the return when someCondition = False will result in an NRE:

bList = myFoo.BarList()
For Each b As Bar in bList ' EXCEPTION
...

Remedy

Replace Exit Function in the function with Return bList. Returning an emptyList is not the same as returning Nothing. If there is a chance that a returned object can be Nothing, test before using it:

bList = myFoo.BarList()
If bList IsNot Nothing Then...

Poorly Implemented Try/Catch

A badly implemented Try/Catch can hide where the problem is and result in new ones:

This is a case of an object not being created as expected, but also demonstrates the counter usefulness of an empty Catch.

There is an extra comma in the SQL (after 'mailaddress') which results in an exception at .ExecuteReader. After the Catch does nothing, Finally tries to perform clean up, but since you cannot Close a null DataReader object, a brand new NullReferenceException results.

An empty Catch block is the devil's playground. This OP was baffled why he was getting an NRE in the Finally block. In other situations, an empty Catch may result in something else much further downstream going haywire and cause you to spend time looking at the wrong things in the wrong place for the problem. (The "silent exception" described above provides the same entertainment value.)

Remedy

Don't use empty Try/Catch blocks - let the code crash so you can a) identify the cause b) identify the location and c) apply a proper remedy. Try/Catch blocks are not intended to hide exceptions from the person uniquely qualified to fix them - the developer.

DBNull is not the same as Nothing

For Each row As DataGridViewRow In dgvPlanning.Rows
If Not IsDBNull(row.Cells(0).Value) Then
...

The IsDBNull function is used to test if a value equals System.DBNull: From MSDN:

The System.DBNull value indicates that the Object represents missing or non-existent data. DBNull is not the same as Nothing, which indicates that a variable has not yet been initialized.

When your DataGridView has AllowUserToAddRows as True (the default), the Cells in the blank/new row at the bottom will all contain Nothing. Most attempts to use the contents (for example, ToString) will result in an NRE.

Remedy

Use a For/Each loop and test the IsNewRow property to determine if it is that last row. This works whether AllowUserToAddRows is true or not:

For Each r As DataGridViewRow in myDGV.Rows
If r.IsNewRow = False Then
' ok to use this row

If you do use a For n loop, modify the row count or use Exit For when IsNewRow is true.

My.Settings (StringCollection)

Under certain circumstances, trying to use an item from My.Settings which is a StringCollection can result in a NullReference the first time you use it. The solution is the same, but not as obvious. Consider:

My.Settings.FooBars.Add("ziggy") ' foobars is a string collection

Since VB is managing Settings for you, it is reasonable to expect it to initialize the collection. It will, but only if you have previously added an initial entry to the collection (in the Settings editor). Since the collection is (apparently) initialized when an item is added, it remains Nothing when there are no items in the Settings editor to add.

If My.Settings.FooBars Is Nothing Then
My.Settings.FooBars = New System.Collections.Specialized.StringCollection
End If

Typically, the Settings collection will only need to be initialized the first time the application runs. An alternate remedy is to add an initial value to your collection in Project -> Settings | FooBars, save the project, then remove the fake value.

Key Points

You probably forgot the New operator.

or

Something you assumed would perform flawlessly to return an initialized object to your code, did not.

Another scenario is when you cast a null object into a value type. For example, the code below:

object o = null;
DateTime d = (DateTime)o;

It will throw a NullReferenceException on the cast. It seems quite obvious in the above sample, but this can happen in more "late-binding" intricate scenarios where the null object has been returned from some code you don't own, and the cast is for example generated by some automatic system.

One example of this is this simple ASP.NET binding fragment with the Calendar control:

<asp:Calendar runat="server" SelectedDate="<%#Bind("Something")%>" />

Here, SelectedDate is in fact a property - of DateTime type - of the Calendar Web Control type, and the binding could perfectly return something null. The implicit ASP.NET Generator will create a piece of code that will be equivalent to the cast code above. And this will raise a NullReferenceException that is quite difficult to spot, because it lies in ASP.NET generated code which compiles fine...

It means that the variable in question is pointed at nothing. I could generate this like so:

SqlConnection connection = null;
connection.Open();

That will throw the error because while I've declared the variable "connection", it's not pointed to anything. When I try to call the member "Open", there's no reference for it to resolve, and it will throw the error.

To avoid this error:

Always initialize your objects before you try to do anything with them.

If you're not sure whether the object is null, check it with object == null.

JetBrains' Resharper tool will identify every place in your code that has the possibility of a null reference error, allowing you to put in a null check. This error is the number one source of bugs, IMHO.

JetBrains' Resharper tool will identify every place in your code that has the possibility of a null reference error. This is incorrect. I have a solution without that detection, yet the code occasionally results to the exception. I suspect it's occasionally undetectable - by them at least - when multithreading is involved, but I can't comment further because I didn't identify the location of my bug yet.
– j rivJan 21 '18 at 7:42

But how to solve it when the NullReferenceException comes in usign HttpContext.Current.Responce.Clear(). It is not getting solved by any of the above solution. because while creating its object object of HttpContext then a error comes "Overload resolution failed because no accessible 'New' accepts this Number of arguments.
– Sunny SandeepFeb 2 '18 at 11:14

It means your code used an object reference variable that was set to null (i.e. it did not reference an actual object instance).

To prevent the error, objects that could be null should be tested for null before being used.

if (myvar != null)
{
// Go ahead and use myvar
myvar.property = ...
}
else
{
// Whoops! myvar is null and cannot be used without first
// assigning it to an instance reference
// Attempting to use myvar here will result in NullReferenceException
}

Be aware that regardless of the scenario, the cause is always the same in .NET:

You are trying to use a reference variable whose value is Nothing/null. When the value is Nothing/null for the reference variable, that means it is not actually holding a reference to an instance of any object that exists on the heap.

You either never assigned something to the variable, never created an instance of the value assigned to the variable, or you set the variable equal to Nothing/null manually, or you called a function that set the variable to Nothing/null for you.

-1: since the question is "What is a NullReferenceException", value types are not relevant.
– John SaundersMay 16 '13 at 22:00

19

@John Saunders: I disagree. As a software developer it is really important to be able to distinguish between value and reference types. else people will end up checking if integers are null.
– Fabian BiglerMay 16 '13 at 22:28

5

True, just not in the context of this question.
– John SaundersMay 16 '13 at 22:44

4

Thanks for the hint. I improved it a bit and added an example at the top. I still think mentioning Reference & Value Types is useful.
– Fabian BiglerMay 16 '13 at 23:02

5

I think you haven't added anything that wasn't in the other answers, since the question pre-supposes a reference type.
– John SaundersMay 18 '13 at 23:24

How profound! I never considered the 'null' constant a reference value. So this is how C# abstracts a "NullPointer" huh ? B/c as I recall in C++, a NPE can be caused by dereferencing an uninitialized pointer (ie, ref type in c#) whose default value happens to be an address that is not allocated to that process (many cases this would be 0, especially in later versions of C++ that did auto-initialization, which belongs to the OS - f with it and die beeotch (or just catch the sigkill the OS attacks your process with)).
– samisJul 31 '13 at 18:55

While what causes a NullReferenceExceptions and approaches to avoid/fix such an exception have been addressed in other answers, what many programmers haven't learned yet is how to independently debug such exceptions during development.

Now, when the NullReferenceException is thrown (or unhandled) the debugger will stop (remember the rule set above?) on the line on which the exception occurred. Sometimes the error will be easy to spot.

For instance,
in the following line the only code that can cause the exception is if myString evaluates to null. This can be verified by looking at the Watch Window or running expressions in the Immediate Window.

var x = myString.Trim();

In more advanced cases, such as the following, you'll need to use one of the techniques above (Watch or Immediate Windows) to inspect the expressions to determine if str1 was null or if str2 was null.

var x = str1.Trim() + str2.Trim();

Once where the exception is throw has been located, it's usually trivial to reason backwards to find out where the null value was [incorrectly] introduced --

Take the time required to understand the cause of the exception. Inspect for null expressions. Inspect the previous expressions which could have resulted in such null expressions. Add breakpoints and step through the program as appropriate. Use the debugger.

1 If Break on Throws is too aggressive and the debugger stops on an NPE in the .NET or 3rd-party library, Break on User-Unhandled can be used to limit the exceptions caught. Additionally, VS2012 introduces Just My Code which I recommend enabling as well.

If you are debugging with Just My Code enabled, the behavior is slightly different. With Just My Code enabled, the debugger ignores first-chance common language runtime (CLR) exceptions that are thrown outside of My Code and do not pass through My Code

where an unboxing conversion (cast) fromobject (or from one of the classes System.ValueType or System.Enum, or from an interface type) to a value type (other than Nullable<>) in itself gives the NullReferenceException.

In the other direction, a boxing conversion from a Nullable<> which has HasValue equal to falseto a reference type, can give a null reference which can then later lead to a NullReferenceException. The classic example is:

Another general case where one might receive this exception involves mocking classes during unit testing. Regardless of the mocking framework being used, you must ensure that all appropriate levels of the class hierarchy are properly mocked. In particular, all properties of HttpContext which are referenced by the code under test must be mocked.

I have a different perspective to answering this. This sort of answers "what else can I do to avoid it?"

When working across different layers, for example in an MVC application, a controller needs services to call business operations. In such scenarios Dependency Injection Container can be used to initialize the services to avoid the NullReferenceException. So that means you don't need to worry about checking for null and just call the services from the controller as though they will always to available (and initialized) as either a singleton or a prototype.

-1: this only handles a single scenario - that of uninitialized dependencies. This is a minority scenario for NullReferenceException. Most cases are simple misunderstanding of how objects work. Next most frequent are other situations where the developer assumed that the object would be initialized automatically.
– John SaundersMar 7 '14 at 0:06

Dependency injection is not generally used in order to avoid NullReferenceException. I don't believe that you have found a general scenario here. In any case, if you edit your answer to be more in the style of stackoverflow.com/a/15232518/76337, then I will remove the downvote.
– John SaundersMar 7 '14 at 0:30

On the matter of "what should I do about it", there can be many answers.

A more "formal" way of preventing such error conditions while developing is applying design by contract in your code. This means you need to set class invariants, and/or even function/method preconditions and postconditions on your system, while developing.

In short, class invariants ensure that there will be some constraints in your class that will not get violated in normal use (and therefore, the class will not get in an inconsistent state). Preconditions mean that data given as input to a function/method must follow some constraints set and never violate them, and postconditions mean that a function/method output must follow the set constraints again without ever violating them.
Contract conditions should never be violated during execution of a bug-free program, therefore design by contract is checked in practice in debug mode, while being disabled in releases, to maximize the developed system performance.

This way, you can avoid NullReferenceException cases that are results of violation of the constraints set. For example, if you use an object property X in a class and later try to invoke one of its methods and X has a null value, then this will lead to NullReferenceException:

public X { get; set; }
public void InvokeX()
{
X.DoSomething(); // if X value is null, you will get a NullReferenceException
}

But if you set "property X must never have a null value" as method precondition, then you can prevent the scenario described before:

I thought to add this as no one mentioned this, and as far as it exists as an approach, my intention was to enrich the topic.
– Nick LouloudakisDec 26 '14 at 1:03

2

Thank you for enriching the topic. I have given my opinion of your addition. Now others can do the same.
– John SaundersDec 26 '14 at 1:05

2

I thought this was a worthwhile addition to the topic given that this is a highly viewed thread. I've heard of code contracts before and this was a good reminder to consider using them.
– VoteCoffeeJan 8 '15 at 2:03

This is incorrect. String.Empty.ToLower() will not throw a null reference exception. It represents an actual string, albeit an empty one (i.e. ""). Since this has an object to call ToLower() on, it would not make sense to throw a null reference exception there.
– KjartanJul 24 '15 at 6:00

Furthermore, the reason I didn't have Html.Partial to begin with was because Visual Studio sometimes throws error-looking squiggly lines under Html.Partial if it's inside a differently constructed foreach loop, even though it's not really an error:

You wanted Html.Partial, not @Html.Partial
– John SaundersJul 24 '15 at 13:55

Also, please show which line threw the exception, and why.
– John SaundersJul 24 '15 at 13:56

The error occurred in MyOtherView.cshtml, which I did not include here, because the Model was not being properly sent in (it was Null), so I knew the error was with how I was sending the Model in.
– Travis HeeterJul 27 '15 at 11:44

As you can see above as with nullable you would access the underlying value through the Value property. Alternatively, you can use an explicit or implicit cast, you can see an example with the return value below:

C# 6.0 introduced the "null-conditional operator" that helps with this a little. With this feature, you can reference nested objects and if any one of them is null the whole expression returns null.

This reduces the number of null checks you have to do in some cases. The syntax is to put a question mark before each dot. Take the following code for example:

var address = country?.State?.County?.City;

Imagine that country is an object of type Country that has a property called State and so on. If country, State, County, or City is null then address will benull. Therefore you only have to check whetheraddressisnull`.

It's a great feature, but it gives you less information. It doesn't make it obvious which of the 4 is null.

Built-in like Nullable?

C# has a nice shorthand for Nullable<T>, you can make something nullable by putting a question mark after the type like so int?.

It would be nice if C# had something like the NotNull<T> struct above and had a similar shorthand, maybe the exclamation point (!) so that you could write something like: public void WriteName(Person! person).

NullReferenceException is meant to be thrown by the CLR. It means that a reference to a null has occurred. It does not mean that a reference to a null would occur except that you cleverly checked first.
– John SaundersMar 7 '16 at 15:43

I see your point about how that would be confusing. I've updated it to a regular exception for this example and a custom exception in GitHub.
– Luis PerezMar 7 '16 at 18:41

Great answer for such a basic question. It's not so bad when it is your code that is failing. It's horrible when it's coming from deep inside some a commercial third party library you are relying on, and the customer support keeps insisting that it has to be your code that is causing the problem. And your not entirely sure it's not and the whole project is ground to halt.. I actually think this might make an appropriate epitaph for my tombstone: "Object reference not set to an instance of an object."
– Darrel LeeMay 3 '16 at 4:01

The error line "Object reference not set to an instance of an object.
" states that you have not assigned instance object to a object reference and still you are accessing properies/methods of that object.

for example: let say you have a class called myClass and it contains one property prop1.

Interestingly, none of the answers on this page mention the two edge cases, hope no one minds if I add them:

Edge case #1: concurrent access to a Dictionary

Generic dictionaries in .NET are not thread-safe and they sometimes might throw a NullReference or even (more frequent) a KeyNotFoundException when you try to access a key from two concurrent threads. The exception is quite misleading in this case.

Edge case #2: unsafe code

If a NullReferenceException is thrown by unsafe code, you might look at your pointer variables, and check them for IntPtr.Zero or something. Which is the same thing ("null pointer exception"), but in unsafe code, variables are often cast to value-types/arrays, etc., and you bang your head against the wall, wondering how a value-type can throw this exception.

(Another reason for non-using unsafe code unless you need it, by the way)

Your dictionary example is not an edge case. If the object is not thread safe, then using it from multiple threads produces random results. Your unsafe code example differs from null in which way?
– John SaundersMar 24 '17 at 2:03

As seen in the above code, the statement
Student s - only declares the variable of type Student, note that the Student class is not instantiated at this point.
Hence, when the statement s.GetFullName() gets executed, it will throw the NullReferenceException.

You are trying to access an object that isn't created or currently not in memory.

So how to tackle this:

Debug and let the debugger break... It will directly take you to the variable that is broken... Now your task is to simply fix this.. Using the new keyword in the appropriate place.

If it is caused on some database commands because the object isn't present then all you need to do is do a null check and handle it:

if (i == null) {
// Handle this
}

The hardest one .. if the GC collected the object already... This generally occurs if you are trying to find an object using strings... That is, finding it by name of the object then it may happen that the GC might already cleaned it up... This is hard to find and will become quite a problem... A better way to tackle this is do null checks wherever necessary during the development process. This will save you a lot of time.

By finding by name I mean some framework allow you to FIndObjects using strings and the code might look like this: FindObject("ObjectName");

If you have a reference to an object, then the GC never cleans it up
– John SaundersDec 24 '15 at 7:51

2

if you use things like FindObject("Name of Object") there is no way GC will know before hand that you are going to refernece that object .. this is what is was trying to explaing .. these occur at runtime
– Akash GuthaDec 24 '15 at 8:11

2

There are some frameworks that Provide this functionality in C# such as Unity . the question has nothing related to BCl. Search the Internet before Criticizing there are a ton of functions like them and for ur kind information i even use it daily. Now please tell me how does the answer doesn't make anysense.
– Akash GuthaDec 24 '15 at 12:35

The examples I saw in your link assign the results of GameObject.Find to a member field. That's a reference and the GC will not collect it until the containing object is collected.
– John SaundersMay 25 '16 at 18:00

Literally the easiest way to fix a NullReferenceExeption has two ways.
If you have a GameObject for example with a script attached and a variable named rb (rigidbody) this variable will start null when you start your game.
This is why you get a NullReferenceExeption because the computer does not have data stored in that variable.

I'll be using a RigidBody variable as an example.
We can add data really easily actually in a few ways:

Add a RigidBody to your object with AddComponent > Physics > Rigidbody
Then go into your script and type rb = GetComponent<Rigidbody>();
This line of code works best under your Start() or Awake() functions.

You can add a component programmatically and assign the variable at the same time with one line of code: rb = AddComponent<RigidBody>();

Further Notes: If you want unity to add a component to your object and you might have forgotten to add one, you can type [RequireComponent(typeof(RigidBody))] above your class declaration (the space below all of your usings).
Enjoy and have fun making games!

To use methods and member of an object you first have to create that object. If you didn't create it (variable that should hold the object is not initialized), but you try to use it's methods or variables you'll get that error.

Sometime you may just forgot to do initialization.

Edited: new can't return null, but fire's exception when failed. Long time ago it was the case in some languages, but not any more. Thanks @John Saunders for pointing that out.

The above code shows simple string which is assigned with a null value.

Now, when I try to print the length of the string str, I do get An unhandled exception of type ‘System.NullReferenceException’ occurred message because member str is pointing to null and there can’t be any length of null.

‘NullReferenceException’ also occurs when we forget to instantiate a reference type.

Suppose I have a class and member method in it. I have not instantiated my class but only named my class. Now if I try to use the method, the compiler will throw an error or issue a warning (depending on the compiler).

Compiler for the above code raises an error that variable obj is unassigned which signifies that our variable has null values or nothing. Compiler for the above code raises an error that variable obj is unassigned which signifies that our variable has null values or nothing.

Why it occurs?

NullReferenceException arises due to our fault for not checking the object’s value. We often leave the object values unchecked in the code development.

It also arises when we forget to instantiate our objects. Using methods, properties, collections etc. which can return or set null values can also be the cause of this exception.

How can it be avoided?

There are various ways and methods to avoid this renowned exception:

Explicit Checking: We should adhere to the tradition of checking the objects, properties, methods, arrays, and collections whether they are null. This can be simply implemented using conditional statements like if-else if-else etc.

Exception handling: One of the important ways of managing this exception. Using simple try-catch-finally blocks we can control this exception and also maintain a log of it. This can be very useful when your application is on production stage.

Null operators: Null Coalescing operator and null conditional operators can also be used in handy while setting values to objects, variables, properties and fields.

Debugger: For developers, we have the big weapon of Debugging with us. If have we face NullReferenceException during the development face we can use the debugger to get to the source of the exception.

In-built method: System methods such as GetValueOrDefault(),IsNullOrWhiteSpace() and IsNullorEmpty() checks for nulls and assign the default value if there is a null value.

There are many good answers already here. You can also check more detailed description with examples on my blog.

You basically copied half of that blog post and added nothing new that existing answers don't address.
– CodeCasterJul 18 '17 at 14:41

@codecaster Is it said copying when you rewrite a summary from your own blog. I know there is nothing new in my answer and nothing new that previous answers dont have but I wish to contribute in more sophisticated way and let others understand the way I understood. Will be glad even if it helps a single person. In good faith.
– WasimJul 18 '17 at 18:01

Beware of classes not instantiating: If any part of your constructor in a class throws a null reference exception the class does not instantiate. In my case it was trying to get a connection string from the web.config that that did not exist.

I instantiated a class:

ClassName myClass = new ClassName();
myClass.RunSomeMethod();

Inside the class itself was a call to get a connection string from the web.config. This part of the constructor threw an null value exception so myClass was null.

If you ever have a situation where a class in not instantiating, try making sure that no part of the class constructor Is throwing a null value exception. F-11 and step through the class and make sure there are no nulls.

Constructors never return null. They either instantiate an object or throw an exception.
– CodeCasterApr 11 '17 at 21:12

@CodeCaster I reworded the entire answer hopefully satistying your concerns. The point being that I haven't seen in other answers is that if any part of the class constructor throws an exception than it will cause the class to not instantiate. Although this makes a lot of sense to me it didn't come as an obvious choice for me.
– logixologistApr 12 '17 at 16:13

Again, this can't happen. If this constructor throws, the next line won't execute.
– CodeCasterApr 13 '17 at 5:32

@logixologist Totally agree with CC here, this answer doesn't make any sense as it stands.
– DavidGJul 5 '17 at 12:36